As shown in FIG. 3, the CCD linear sensor has channel regions 12 separated by channel stopper regions 11 which are formed by P-type impurity diffusion. Read-out gate electrodes 13 (13a, 13b) and shift registers 14 (14a, 14b) are formed common to the respective channel regions 12. Particularly, part of each of the channel region 12 is covered by a light-shielding film 15 so that a photosensitive region 16 is formed within each of the channel region 12.
In the charge accumulation period, signal charges e corresponding to image information are accumulated in each of the photosensitive regions 16. In the next reading-out period, the signal charges e accumulated in, for example, odd-numbered photosensitive regions 16 are transferred through one read-out gate 13a to one shift register 14a. The signal charges e accumulated in the even-numbered photosensitive regions 16 are transferred through the other read-out gate 13b to the other shift register 14b. The signal charges e transferred to the shift registers 14a and 14b are further transferred in the horizontal direction, i.e. to an output circuit (not shown) by a two-phase transfer pulse.
When the signal charges are transferred from the photosensitive regions 16 through the read-out gates 13 to the shift registers 14, the transfer efficiency for each photosensitive region 16 must be substantially equal. Therefore, in the conventional CCD linear sensor, the channel regions 12 are formed uniformly square, for example, rectangular in their surfaces as illustrated.
FIG. 4 shows the potential diagram taken along a line II--II in FIG. 3. It will be seen that the potential distribution in the channel region is substantially flat since the surface shape of the channel region 12 is rectangular. Therefore, when the reading-out operation is performed at a higher frequency, part of the signal charges remains at the fore end (or at a in FIG. 4) of the photosensitive region 16. That is, so-called transfer residue occurs thereat.
Moreover, at a portion in the channel region where reading-out electric field is minimum, or at the intermediate portion (as represented by b in FIG. 4), the potential distribution is flat. Besides, the flat portion b is long. Therefore, in the intermediate portion b, the signal charges e are transferred at a slow speed. As a result, a frequency (sampling frequency) of a pulse to be applied for read-out driving is limited.
For this reason, upon read-out driving, the conventional CCD linear sensor cannot increase the transferring speed of the signal charges e even by applying a read-out pulse of a high sampling frequency. Consequently, the speed-up of reading-out operation cannot be realized.
As shown in FIG. 5, a CCD linear sensor is proposed in which the width D of all the channel regions 12 is continuously widened over the whole of each channel region 12 toward the shift registers 14.
FIG. 6 shows the potential diagram taken along a line III--III in FIG. 5. It will be seen that a potential gradient locally occurs at the fore end a of the photosensitive region 16 and at a portion (c in FIG. 6) of the channel region 12 corresponds to the read-out gate 13a. However, the intermediate portion b of the channel region 12 still has a substantially flat potential distribution.
Therefore, even in the CCD linear sensor shown in FIG. 5 and 6, the transfer operation for the signal charges e is limited in the intermediate portion b of the channel region 12. Thus, even if a read-out pulse of a high sampling frequency is applied, the transfer speed for the signal charges e will not be increased. In addition, since the fore end a of the photosensitive region 16 coincides with that of the channel region 12, a sharp gradient of the potential distribution occurs at the fore end a of the photosensitive region 16. Thus, part of the signal charges e are not accumulated at the fore end a of the photosensitive region 16. It causes reduction of the opto-electro conversion efficiency at the photosensitive region 16.